In the production of elongate hollow bodies, for example pipes or tubes or similar parts, which are of a complicated configuration and which more especially involve three-axis curvature, using an extrusion blow molding procedure, a typical mode of operation involves the preform which is generally of a tubular nature being introduced into the mold cavity of a mold portion which is disposed at least in part in a horizontal position, of a blow molding mold having two or more portions, in such a way that prior to the commencement of the blow molding operation the preform, following the shape of the mold cavity which is of a correspondingly complicated configuration to that required for the hollow body to be produced, at least over large parts of the longitudinal extent of the preform, is disposed within the mold cavity, that is to say between the two boundary edges extending in the longitudinal direction of the mold cavity, and thus the preform does not involve a larger diameter than corresponds to the width of the mold cavity. That arrangement and relationship provide that, when the blow molding mold is closed by the two or more mold portions being moved towards each other, no or only a little surplus material is squeezed off the preform or the hollow body to be produced therefrom, that is to say along the longitudinal extent of the lateral boundary edges of the mold cavity. That avoids the occurrence of waste pieces and also weak points on the finished hollow body.
After the blow molding mold is closed the preform is expanded by an increased internal pressure therein in the usual manner, but in the production of tubular hollow bodies the extent of that expansion of the preform is generally not very great.
The operation of fitting the preform into the mold cavity is usually implemented in such a way that either the extruder with the extrusion head that produces the preform is moved with respect to the stationary mold portion which receives the preform, with such movement being adapted to the configuration of the mold cavity so that the preform is suitably laid therein, or the mold portion is moved in a horizontal plane with respect to the extrusion head which is movable vertically, that is to say which can be lowered, in order to lay the extruded preform into the mold cavity. Both cases involve moving considerable masses, with the consequence that the drive arrangements required for that purpose are also of a correspondingly complicated and expensive nature.
Reference may be made at this point to DE 43 05 735 C1 disclosing an operating procedure whereby, in an extrusion blow molding apparatus for the production of multi-axis hollow bodies of plastic material, arranged between the extrusion head of the apparatus and the blow molding mold is a preliminary mold which is adapted to the configuration of the mold cavity of the blow molding mold and which has an open mold trough member which, at least in the tubular preform-receiving position thereof is displaceable under the extrusion head in two horizontal directions and can be raised and lowered in a vertical direction. That preliminary mold firstly receives the preform in order then to discharge it to the mold cavity of the blow molding mold portion which is disposed in a horizontal position. That design configuration admittedly avoids movement of the heavy blow molding mold and movement of the extruder in a horizontal plane, but for that purpose the apparatus requires the special mold trough member referred to above, which in addition must be so designed that, when the preform is transferred into the mold cavity of the blow molding mold, the preform always remains in the correct position and is not subjected to any unacceptable loadings.